The present invention relates to high intensity light sources in Fiber Optic Gyroscopes (referred to herein as FOGs), and more particularly, to methods of and systems for stabilizing, over a wide temperature range, the output characteristics of a superluminescent diode.
FOG measurement error can generally be divided into the categories of bias error and scaling error. All gyroscopes have a certain degree of measurement error that is present upon initialization, referred to as bias error. The second category of error, referred to herein as scaling error (or alternatively scale factor error), accumulates over the angle through which a gyroscope is being rotated. Scaling error is the difference between the actual angle of rotation the FOG experiences and the angle of rotation indicated at the FOG output. A FOG indicating that it had turned ninety degrees when it had, in fact, turned ninety-two degrees, is an example of scaling error. The amount of scaling error may be affected by various environmental factors, so that a fixed compensation value is generally not sufficient to completely correct the FOG output.
FOGs typically use superluminescent diodes (referred to herein as SLDs) as light sources. The performance of a FOG is dependent upon the wavelength of the light source, since the scaling error associated with the FOG is directly proportional to the wavelength of the light from the SLD. The wavelength of the SLD varies linearly with its operating temperature, so it is necessary to temperature-stabilize the SLD with a thermoelectric cooling module (TEC) in order to limit scale factor variations over a wide range of FOG operating temperatures.
Commercially available (prior art) SLD devices 10 typically include a TEC component 12 mounted internally within the SLD device package 14, as shown in block diagram form in FIG. 1. The SLD chip 16, the light emitting element, is mounted directly to the surface of the TEC 12 along with a thermistor 18. The thermistor leads 20 are brought out from the package to permit operation with external temperature control electronics 22.
The wavelength (and consequently scale factor) sensitivity of the SLD is typically on the order of 400 parts per million (ppm) per degree Celcius (° C.). In order to limit scale factor variations to within an exemplary target goal of 100 ppm, it is necessary to control the temperature of the SLD to within at least 0.25° C. over the operating temperature range. A typical operating temperature range is from −54° C. to 71° C. The temperature control electronics combined with an ideal TEC would be able to provide the control functions necessary for this level of stability. However, limitations of the construction and performance of commercially available TEC modules preclude scale factor stability better than 100 ppm.